Predicting Project length of time

I've only dabbled in Rapidminer a bit and don't have a data mining background so I'm looking to get some direction/help on this. I'd like to predict how long a project will take to complete once it's handed off from sales to project management. Historically, this time can range from 14 days to well over 300 days based on a number of factors.

In my current model, my label is Days from Contract Signing to Project Complete and is numeric. I have about 20 other attributes in my dataset that I've included for training, though I'm not sure I'm using them correctly. I'm using a Deep Learning operator, but my RMSE is too high.

My goal is to predict a five-day window of when the project should be completed based on the 20 attributes mentioned above. Is there a better way to accomplish this?